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8883 



Bureau of Mines Information Circular/1982 



Vibration Qualification of Electronic 
Instrumentation for Underground 
Coal Mining Machinery 



By Roy C. Bartholomae, Bruce S. Murray, 
and Richard Madden 




UNITED STATES DEPARTMENT OF THE INTERIOR 



obM* 



Information Circular 8883 



Vibration Qualification of Electronic 
Instrumentation for Underground 
Coal Mining Machinery 



By Roy C. Bartholomae, Bruce S. Murray, 
and Richard Madden 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 










This publication has been cataloged as follows: 



Bartholomae, R. C 

Vibration qualification of electronic instrumentation for 
underground coal mining machinery. 

(Information circular ; 8883) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:8883. 

1. Coal-mining machinery— Vibration— Measurement. 2. Electronic 
apparatus and appliances— Vibration— Testing. I. Murray, Bruce S. 
II. Madden, Richard. III. Title. IV. Series: Information circular 
(United States. Bureau of Mines) ; 8883. 

TN295.U4- [TN813] 622s [622] 82-600497 AACR2 



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CONTENTS 

Page 

Abstract 1 

Introduction 2 

Data base 2 

Data analysis 3 

Development of vibration qualification tests 6 

Discussion 9 

ILLUSTRATIONS 

1. Horizontal acceleration on main frame of continuous miner during loading... 2 

2. Acceleration level distribution in the 160-Hz 1/3-octave band 4 

3. Cumulative probability of acceleration level in 160-Hz 1/3-octave band 4 

4. Summary of statistics of acceleration levels for underground mining 

machines 6 

5. Summary of swept sine vibration qualification tests issued by the military. 8 

6. Comparison of underground mining machine vibration data with MIL-STD-810B. . 8 

TABLE 

1. 1/3-octave band acceleration level statistics 5 



' 



Bnona 



VIBRATION QUALIFICATION OF ELECTRONIC INSTRUMENTATION 
FOR UNDERGROUND COAL MINING MACHINERY 

By Roy C. Bartholomae, ! Bruce S. Murray, 2 and Richard Madden 3 



TJ-i 



ABSTRACT 

An accurate characterization of the vibration environment and a 
vibration qualification test derived from it will be a very useful tool 
for manufacturers of instrumentation for use on underground coal mining 
equipment. Recognizing this, the Bureau of Mines sponsored a study 
wherein vibration levels were measured on mining equipment to form a 
basis for developing the required vibration test. The data base was 
composed of 160 samples taken at different positions on a variety of 
underground machinery. The data were analyzed and presented in a format 
typical of military vibration qualification tests. The form was shown 
to be virtually identical to the swept sine test envelope specified in 
MIL-STD-810B for tracked vehicles. 



Supervisory electrical engineer, Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, Pa. 

■'Senior engineer, Bolt Beranek and Newman, Inc., Cambridge, Mass. 
3 Department manager, Bolt Beranek and Newman, Inc., Cambridge, Mass. 



INTRODUCTION 



The desire for increased production 
of coal, together with the growing empha- 
sis on health and safety in underground 
coal mines, means that additional elec- 
tronic instrumentation will be needed on 
underground mining machines. To insure 

operation of such 
the harsh environ- 



long-term reliable 

instrumentation in 

ment of underground 

must consider the 

at the design stage. 

the instrument designer is a vibration 



mines, manufacturers 

effects of vibration 

A useful tool for 



qualification test, preferably one de- 
signed to meet an existing military stan- 
dard, because many commercial labora- 
tories are already set up to perform 
testing to meet military standards. 

This Bureau of Mines report de- 
scribes the development of a vibration 
qualification test and compares the test 
with similar ones used by the armed 
forces. 



DATA BASE 



The first step in developing the 
vibration qualification test was to cre- 
ate a data base. Under U.S. Bureau of 
Mines Contract H0155113, data were taken 
on 30 mining machines of 8 different 
types, including continuous miners, 
loaders, cutting machines, track jeeps, 
face drills, shuttle cars, roof bolters, 
and scoop trams. Vibration measurements 



were made at a variety of positions on 
each machine, resulting in a total of 
160 samples in the data base. Data were 
recorded on magnetic tape and then ana- 
lyzed in 1/3-octave bands in the fre- 
quency range between 3.2 and 500 Hz. A 
typical sample of data taken on a conti- 
nuous miner is shown in figure 1. 



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3.15 6.3 12.5 25 50 100 200 400 

ONE-THIRD OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 1. - Horizontal acceleration on main frame of continuous miner during loading. 



800 



DATA ANALYSIS 



Data from each of the samples were 
combined to form a 160-sample data set in 
each of the 23 1/3-octave bands between 
3.2 and 500 Hz. The following analysis 
was used to convert the data base into a 
usable vibration qualification test. 

Qualification tests are customarily 
based upon the highest vibration levels 
encountered in service. The highest 
level can be estimated as that level 
which is not exceeded either more than 
1 time in 100 (99 +h percentile) or more 
than 1 time in 1,000 (99.9 +h percentile). 
Our 160-sample data base is clearly too 
small in itself to determine these low- 
probability occurrences. If, however, 
the 160-sample data base is representa- 
tive of a larger well-defined data base, 
we can determine either or both of these 
percentiles with confidence. Fortu- 
nately, we are able to show that a normal 
distribution curve well represents the 
data. We illustrate this by constructing 
an amplitude histogram from the samples 
in each 1/3-octave band and comparing it 
to the normal distribution. The histo- 
gram was formed by grouping the data in 
ten 5-db steps in the acceleration level 
range from -40 db re 1 g (0.01 g) 4 to +5 
re 1 g (1.78 g) . 

The total number of samples in each 
histogram equals the total number of re- 
cordings (that is, 160). In cases where 



^Acceleration level 
(acceleration in g's). 



data fell below the noise level of the 
measurement system, these data were 
grouped with the -40 db re 1 g (0.01 g) 
data. 

As an example, the histogram for the 
160-Hz 1/3-octave band is presented in 
figure 2. The number of occurrences in 
each 5-db step is given by the ordinate 
to the left, and the associated probabil- 
ity of occurrence is given on the right. 

The cumulative percentage distribu- 
tion is plotted on normal distribution 
paper in figure 3. Since a straight line 
fits the data very well, the data can be 
considered normal. Note that deviations 
from the line at the end points are not 
significant; that is, at lower levels, 
data below the noise floor were grouped 
with the -40 db re 1 g data, and at the 
high end there is only one sample greater 
than db re 1 g. The mean for this sam- 
ple is given by the 50 +h percentile as 
-22.5 db, and the standard deviation is 
10.5 db. 

Extrapolation based upon a normal 
distribution with a mean of -22.5 db re 
1 g and a standard deviation of 10.5 db 
gives the 99 +h and 99.9 +h percentiles 
(rounded to the nearest 0.5 db) as 

ALg9 # 9 = P + 3.1a = 10 db re 1 g 

and AL99 = \i + 2.33a = 2 db re 1 g. 



equals 20 1og<|Q 






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ACCELERATION LEVEL, db re 1 g rms 

FIGURE 2. - Acceleration level distribution in the 160-Hz 1/3-octave band. 




-30 -20 -10 

ACCELERATION LEVEL, db re 1 g rms 

FIGURE 3. - Cumulative probability of acceleration level in 160-Hz 1/3-octave 



band. 



A similar analysis was performed for 
data in each of the 1/3-octave bands, and 
in each case the distribution was normal. 
The mean, standard deviation, and 99 th 
and 99.9 +h percentile values for each 
1/3-octave band are listed in table 1 and 
plotted in figure 4. It is important to 
remember that these values describe the 
statistics of the samples from each 
1/3-octave band and that they do not 
describe an expected spectrum shape. 



Rather, the data provide an overall enve- 
lope upon which to base qualification 
tests. Also plotted in figure 4 are the 
maximum values recorded in each of the 
1/3-octave bands. Note that a majority 
of these values fall between the 99 th 
and the 99.99 th percentile values, as 
expected from the 160-point sample size, 
thus providing a limited validation of 
the approach. 



TABLE 1. - 1/3-octave band acceleration level statistics 



1/3-octave band 


Acceleration level, db re 


1 g rms 


center frequency, 


Mean 


Standard 


99th 


99.9th 


Hz 




deviation 


percentile 


percentile 


3.2 


-18 


9.5 


+4 


+11.5 


4.0 


-20 


9 


+1 


+8 


5.0 


-21 


11 


+4.5 


+12 


6.3 


-21.5 


11.5 


+5.5 


+14 


8.0 


-23.5 


8.5 


-4 


+2.5 


10 


-25 


10.5 


-.5 


+7.5 


12 


-25 


9.5 


-.5 


+4.5 


16 


-26 


10.5 


-1.5 


+6.5 


20 


-25 


9.5 


-3 


+4.5 


25 


-23.5 


8.5 


-1.5 


+3 


32 


-24 


10 


-.5 


+7 


40 


-24.5 


13 


6 


+15 


50 


-24 


11 


+1.5 


+10 


63 


-25 


12.5 


+4 


+13.5 


80 


-23.5 


11 


+2 


+10.5 


100 


-23.5 


10 





+7.5 


125 


-22.5 


10 


+1 


+8.5 


160 


-22.5 


10.5 


+2 


+10 


200 


-22 


10 


+1 


+9 


250 


-20 


10 


+3.5 


+11 


315 


-17 


11.5 


+10 


+18.5 


400 


-17.5 


12 


+10.5 


+19.5 


500 


-17.5 


11.5 


+9.5 


+18 





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Highest measured levels 




Mean 



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ONE-THRID OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 4. - Summary of statistics of acceleration levels for underground mining machines. 



800 



DEVELOPMENT OF VIBRATION QUALIFICATION TESTS 



Virtually all the vibration qualifi- 
cation tests conducted in the United 
States are based on Military Standards 
(MIL-STD) developed over many years. 
These standards reflect experience in a 
vast number of environments. It is rea- 
sonable, therefore, to base any vibration 
tests upon the military standards, at 
least initially, and amend them later if 
necessary to meet particular require- 
ments. Another major benefit of the use 
of military standards relates to test 
equipment performance and limitations. 



The development of these standards as 
viable test procedures has been closely 
linked with the attainable performance of 
readily available shakers and associated 
hardware. Thus, specification of MIL-STD 
levels will assure that the tests are 
experimentally possible. 

With these considerations in mind, 
we reviewed four pertinent military stan- 
dards to determine their suitability 
for the underground mining equipment. 



The vibration qualification standards 
reviewed were MIL-STD-167, MIL-STD-810B , 
MIL-STD-810C, and MIL-E-5272-C . 

Although many other standards in- 
clude vibration test procedures, these 
four provided data for a wide range of 
applications. MIL-STD-167 provides data 
on shipboard vibration levels; MIL- 
STD-810 is the U.S. Air Force environmen- 
tal test procedure for flight and ground 
vehicles; and MIL-E-5272-C is concerned 
with environmental testing of equipment 
destined for aircraft, ship, and missile 
applications. 

The review consisted of evaluating 
the vibration test specifications in each 
of the Military Standards against the 
measured and predicted vibration levels 
of underground mining equipment. Fig- 
ure 5 presents a summary of the vibra- 
tion spectra specified for swept sine 
tests in the MIL-STD's reviewed. The 
spectra shown for MIL-E-5272-C and MIL- 
STD-167 are the highest levels indicated 
in that particular specification. How- 
ever, in the case of MIL-STD-810B and 
MIL-STD-810C, we have presented the most 
appropriate spectrum based on the 
description in the standard "tracked 
vehicles." Note that these spectra are 
presented with the measure of vibra- 
tion amplitude being "displacement (peak- 
to-peak) inches" rather than "db re 1 g 
rms. " 

The basic expression used to con- 
vert vibration amplitude from acceler- 
ation (in db re 1 g rms) to displacement 



(in inches peak to peak), assuming that 
the vibration is sinusoidal, is given by 



D.A. - g*$ 1C/20 

where D.A. = displacement (peak-to-peak), 
inches, a = acceleration (db re 1 g rms), 
f = frequency (Hz), and g = 386.4 
(in/sec 2 ). 

This expression reduces to 

27.68 



D.A = 10 a / 20 x 



77- 



As an example, we calculate the dis- 
placement produced by the 99.9 th percen- 
tile acceleration at 160 Hz from the data 
in table 1: 



D.A. = 1010/20 x 



27.68 
(160)2 



= 0.0034 in. 



Figure 6 presents a summary of the 
vibration levels expected on underground 
mining machines, the displacement values 
being derived from the data given in 
table 1. The open circles represent the 
vibration amplitude that will only be 
exceeded 1 time in 1,000 and the closed 
circles show the amplitudes that will be 
exceeded 1 time in 100. We have also 
plotted the MIL-STD-810B level for 
tracked vehicles, and it is seen to be a 
good match over virtually all the fre- 
quency range except below 8 Hz . The 
other specifications plotted in figure 5 
do not provide a suitable match to the 
plotted points. 



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DISCUSSION 



The deviation of the standard below 
8 Hz is most likely due to the normal 
displacement limitations of commercial 
shakers. This is an example of a speci- 
fication being tailored to fit the avail- 
able equipment so that the testing can 
be performed economically. It is likely 
that the tracked vehicles considered 
by MIL-STD-810B have low-frequency vibra- 
tion amplitudes much like those plotted 
in figure 6. Since the displacement- 
limited curve satisfactorily tests the 
equipment of these vehicles, we may con- 
clude that it will do the same for the 
equipment on underground mining machin- 
ery. Therefore, we recommend that the 
MIL-STD-810B vibration tests curves for 



category f equipment 
curve W. 



be employed, using 



It should be noted that MIL-STD- 
810B, issued in June 1967, is not the 
latest issue of this standard; it is used 
because MIL-STD-810C, issued in 1975, 
specifies a different vibration test for 
components on tracked vehicles that does 
not adequately simulate the expected 
levels on mining equipment. The specifi- 
cation set in issue B is equivalent to a 
±4 g level from 9 to 500 Hz. The speci- 
fication in issue C is ±1.5 g from 5.5 to 
30 Hz and ±4.2 g from 50 to 500 Hz. The 
reason for the reduction in levels below 
50 Hz is not known. 



INT.-BU.OF MINES, PGH..PA. 26133 







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